Around us, there are many articles, such as transformers (transformers), electric motors (motors), generators, speakers, induction heaters and a variety of actuators, which utilize electromagnetism. In order to make them high-performance and downsize them, it is indispensable to improve the performance of permanent magnets (hard magnetic substances) and soft magnetic materials. Hereinafter, among these magnetic materials, magnetic cores (magnetic cores), one of soft magnetic materials, will be hereinafter described.
When magnetic cores are disposed in magnetic fields, it is possible to produce large magnetic flux densities, and accordingly it is possible to downsize electromagnetic appliances and improve the performance. Naming a specific example, magnetic cores are used in order to enlarge local magnetic flux densities by fitting them into electromagnetic coils (hereinafter, simply referred to as coils), or to form magnetic circuits by intervening them in a plurality of coils.
Such magnetic cores are required to exhibit a large magnetic flux in order to enlarge magnetic flux densities, and simultaneously to exhibit a less high-frequency loss (iron loss) because they are often used in alternating magnetic fields. As the high-frequency loss, there are hysteresis loss, eddy current loss and residual loss, however, the hysteresis loss and the eddy current loss matter mostly. The hysteresis loss is proportional to the frequency of alternating magnetic fields, on the other hand, the eddy current loss is proportional to the square of the frequency. Accordingly, when they are used in high-frequency ranges, it is especially required to reduce the eddy current loss. In order to reduce the eddy current loss, it is needed to reduce currents which flow into magnetic cores by induction electromotive forces, to put it differently, it is desired to enlarge the specific resistance of magnetic cores.
Conventional magnetic cores have been manufactured by laminated silicon steel while intervening insulative layers therebetween. In this case, it is difficult to manufacture small magnetic cores, moreover, the eddy current loss is still large because the specific resistance is small. Hence, as magnetic cores whose formability is improved, magnetic cores are used in which iron-based powders are sintered. However, since the magnetic cores exhibit a small specific resistance, they are mainly used in DC coils, and are less likely to be used in AC coils. Moreover, in order to enlarge the specific resistance, it is disclosed in PCT International Laid-Open Publication No. 2000-504,785 and the like to manufacture a magnetic core by high-pressure forming an iron-based magnetic powder covered with an insulation film. When this iron-based magnetic powder is used, since it is good in terms of the formability, and simultaneously since the respective particles of the powder are covered with the insulation film, a magnetic core with a large specific resistance is obtained. Hereinafter, magnetic cores which are made by pressure forming iron-based magnetic powders thus covered with insulation films will be referred to as “powder magnetic cores.”
Thus, the powder magnetic cores exhibit a large specific resistance, and exhibit a large degree of configuration freedom, however, the conventional powder magnetic cores have a low density and the magnetic characteristics, such as the magnetic permeability, are not necessarily sufficient. Of course, it is possible to highly densify the powder magnetic cores by enlarging the compacting pressure, however, it has been difficult inherently to enlarge the compacting pressure. Because, when the compacting pressure is enlarged to high pressures, galling occurs on the surface of dies so that dies are impaired and the surface of powder magnetic dies is bruised, and moreover ejecting forces are enlarged so that it has become difficult to eject powder magnetic cores. Such assignments are detrimental when considering industrial mass-production.
Note that, in view of known literatures, there might exist descriptions and the like to the effect that high-pressure forming is possible, however, highly densifying powder magnetic cores, improving the magnetic characteristics and the like have not been accomplished actually so far by that means.